Magnetic core



July 31, 1951 2 Sheets-Sheet 1 Filed Dec. 31, 1948 "a a m pm. 2 or w *W 9 m MM W 2 "CM y 2 b F Z a u a U Q P His At tovn ey.

July 31, 1951 w. w. BROOKS 2,562,693

MAGNETIC CORE Filed Dec. 31, 1948 2 Sheets-Sheet 2 Fig.7.

Inventor: Wes\e W- Bvooks,

His AtbOTney.

Patented July 31, 1951 MAGNETIC CORE Wesley W. Brooks, Fort Wayne, Ind., assignor to General Electric Company, a corporation oi New York Application December 31, 1948, Serial No. 68,546

18 Claims. 1

This invention relates to magnetic cores and more particularly to improvements in cores for stationary induction apparatus.

In many types of magnetic cores, it is necessary to provide gaps in a magnetic circuit which are either complete breaks or gaps or else are socalled bridged gaps in which, in'efl'ect, there is a gap which is magnetically paralleled by a restricted section of the core which has a higher flux density than the rest of the core. Such gaps are used to control the exciting current of induction apparatus or the distribution of the flux in various parts of the core and, due to magnetic saturation in the bridging parts of a bridged gap, to control also the rate 01' change of flux in certain parts of the core relative to the flux rate of change in other parts thereof so as to provide certain desirable voltage changes in the winding, linking various parts 01' the core. A novel and simple core structure in which such gaps are provided for the control of exciting current or the distribution of flux in various parts of the core is disclosed in patent application, Serial No. 788,063, filed November 25, 1947, now Patent 2,553,554, issued May 22, 1951, in the name of Roy H. Dierstein and assigned to the same assignee as the present invention. The present invention discloses an improved magnetic core structure of this type for stationary induction apparatus which has particular application to the construction of cores for fluorescent lamp.

ballasts though it is in no way limited thereto.

An important consideration in the construction of fluorescent lamp ballasts is the degree of hum or noise level associated with the device which results from the relatige movement of the laminations comprising the magnetic core and from vibrations set up in neighboring magnetic material due to stray leakage flux. This noise is due to the cyclic mechanical stresses set up within the magnetic material as a result of the alternating supply current. When applied to normal lighting circuits, the 60-cycle hum due to the presence or the fluorescent lamp ballast may, if the noise level is high enough, be extremely disagreeable and may negatively affect the efficiency of personnel working in the vicinity of such ballasts. Thus, one of the primary considerations in the construction of a fluorescent lamp ballast is the reduction 01' noise level.

Therefore, it is an object of this invention to provide a new and improved magnetic core construction for stationary induction apparatus which results in the reduction of the noise level or hum of such alternating current devices.

It is a further object of this invention to provide a new and simple magnetic core construction for high reactance transformers which has a plurality of magnetic gaps.

This invention will be better understood from the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a sectional view of a two-lamp fluorescent ballast having a core constructed in accordance with this invention. Fig. 2 is a plan view of a modified construction of a magnetic core for a two-lamp fluorescent ballast. Figs. 3, 4 and 5 are plan views of modified portions of magnetic cores constructed in accordance with this invention. Fig. 6 is a sectional view of a preferred construction of a two-lamp fluorescent ballast having a core prepared in accordance with this invention. Fig. 7 is an enlarged view of a portion of magnetic core shown in Fig. 6. Fig. 8 is a circuit diagram showing the application of a fluorescent lamp ballast to a two-lamp fluorescent lighting circuit.

Referring now to the drawing, and more particularly to Fig. 1, there is shown therein a twolamp fluorescent ballast 1 whose core comprises a generally straight winding leg 2 and oppositely disposed multi-legged yoke members 3. The core is constructed of a plurality of laminations stacked one above the other, each laminar layer being made up of a winding leg 2 and two yoke members 3. The yoke members 3 have end legs 4 and-5 and intermediate leakage flux legs 6 and l. Wound on the central core leg 2 are three coils 8, 9 and in so oriented that coils 8 and 9 are located between yoke legs 4 and 6 and l and 5, respectively. The coils 8 and 9 represent the secondary windings of a transformer whose primary winding 10 is mounted on the central core leg 2 between magnetic shunt legs 6 and 1 of the yoke members 3.

The ballast i many be clamped together as a unit by any suitable means, such as the spring clamp H which is similar in construction to the novel clamping means disclosed in patent application, Serial No. 24,047, filed April 29, 1948, inv

the name of Gregory J. Ganley and assigned to the same assignee as the present invention.

The windings 8, 9 and Ill are shown diagrammatically in Fig. 8. As is often the case in twolamp fluorescent circuits, one lamp l2 and a secondary coil (in the illustrated case coil 9) are operated with leading current which is provided by a series capacitor l3, and the second lamp I4 and the remaining secondary winding (in the and lagging secondary flux with relation to-the primary or exciting flux. Thus, as the secondary winding 9 carries leading current, the flux in that part of the winding leg is more or less in phase with the exciting flux produced by the primary winding 10 so that the vector difference between these two fluxes, which is the only flux which flows in the yoke leg 1, is relatively small. Likewise, as the coil 8 carries lagging current, the flux produced by this winding is more or less in phase opposition to the exciting flux induced in the central part of the winding leg by the primary coil 10, and, consequently, the vector difference between these two fluxes is relatively large and it is this vector resultant of flux which flows in the yoke legs 6. Consequently, in order to maintain approximately equal flux density in the two leakage path yoke legs 6 and 1, the le 6 must have a substantially greater area. than the leg 1.

In order to prevent the leakage flux paths from having too low a reluctance, it is usually necessary to provide them with a high reluctance section, such as an air gap, and it has also been found that desirable results are obtained when the portion of the core which is traversed by the leading secondary flux is also provided with a high reluctance section, preferably a bridged gap. This latter feature forms the subject matter of an application, Serial No. 731,559, filed February 28, 1947, in the name of Harold W. Lord and assigned to the present assignee. In Fig. 1, these various air gaps are obtained by properly shaping the winding leg punching 2 and the outer yoke legs 5. Thus, it will be observed that there are depressions or notches 15 in the sides of the winding leg 2 in the vicinity of the ends of the yoke legs 6 so as to form air' gaps. Similarly, the

air gaps are formed between the winding leg 2 and the ends of the yoke legs I by the notches or depressions 16.

In order to achieve a bridged gap in the portion of the core which is traversed by the leading secondary flux, a cylindrical pin 11 is forced into cooperating grooves 18 and 19 formed in the central winding leg 2 and in the end yoke legs 5, respectively. Thus, the movement of the winding leg 2, with respect to the yoke legs 3, is minimized, and air gaps located between the end of the central core leg 2 and the outer yoke legs 5 are maintained by the mechanical interference of the pin 11.

In the previously mentioned patent application, Serial No. 788,063, filed November 25, 1947, in the name of Roy H. Dierstein and assigned to the same assignee as the present invention, there is described core constructions which are applicable to use in the construction of fluorescent lamp ballasts. In one core construction disclosed in that application, air gaps exist between the center core leg and the outer yoke legs at one end of the core. Therefore, flux traversing these air gaps may leak out into the surrounding space and, upon contacting magnetic material in the near vicinity, may cause excessive noise or heating or both.

To partially eliminate this, an alternative core construction is illustrated in the above-mentioned application which provides a magnetic bridge between the two yoke members and the central core leg. This bridge is located on the periphery of the core, and the air gap is located within the core structure. Even though this restricts a large portion of the flux and prevents it from leaking into the adjoining areas, there may still be sufflcient leakage of flux into surrounding magnetic materials to cause objectionable effects. A means of overcoming this flux leakage is to provide a semi-forced core construction similar to that shown in Patent No. 1,783,050, issued to Wayne J. Morrill and assigned to the same assignee as the present invention, wherein the central core leg is completely surrounded by the yoke member. The construction disclosed in the above-mentioned patent presents difficulties in assembly and fabrication which would, at the present time, make the flnal manufactured article prohibitively expensive.

Upon examination of Fig. 1, it can be seen that the central core leg 2 is substantially completely surrounded by the yoke members 3, thus preventing the leakage of flux into surrounding magnetic materials. The noise due to the presence of leakage flux in nearby magnetic material is nearly entirely eliminated by this construction.

The problem now remains to eliminate ballast noise due to the vibration or oscillation of the laminar layers of the magnetic core. In the magnetic core construction disclosed in the previously mentioned Dierstein patent application, the lead end air gap and lag end joint are located on the sides of the central core leg. It was unexpectedly discovered that by moving the lead end gap and the lag end joint from the sides of the winding leg to its ends, a considerable reduction in the vibratory noise level of the fluorescent ballast resulted, thus attaining the quietness of the Morrill forced core construction and the cost reduction of the Dierstein split yoke construction.

However, the expense involved in the additional operation of inserting the pin 11 in Fig. 1 is objectionable. To eliminate the necessity of usin pin 11, studs 20 are formed integral with and as extensions of the outer legs 2| of the yoke members 22, as shown in Fig. 2. The studs 20 mechanically interfere with the central winding leg 23 so as to produce essentially the same core as illustrated in Fig. 1. It should be noted that even though there is an air gap separating the two yoke members, as can be seen in both Fig. 1 and Fig. 2, there will be little or no flux in this area and the leakage of flux into surrounding space is minimized since there is no magnetic potential acting across the air gap.

Figs. 3, 4 and 5 illustrate alternative constructionsof the magnetic core which incorporates a plurality of studs either in the outer yoke leg or incorporated in the construction of the central winding leg. Thus, the studs 24, as shown in Fig. 3, of the end legs 25 rest midway along the diagonal end sides 26 of the central winding leg 21; the studs 28, as shown in Fig. 4, of the outer yoke legs 29 rest against the tip 30 of the central core leg 21 which is the vertex formed by the intersection of the diagonal end sides 26; the studs 3|, as shown in Fig. 5, are incorporated as part of the central winding leg 32 and are formed in the diagonal end sides so as to mechanically butt against the outer yoke legs 33.

The constructions shown in Figs. 2 through 5 require a plurality of studs to achieve the semiforced core effect and high accuracy of the dies involved in the punching operation. Shown in Fig. 6 is a fluorescent lamp ballast 34 which is constructed in accordance with this invention and members 43 which are oppositely disposed on either side of the central winding leg 35. As in Fig. l, the coils 38 and 31 represent the secondary windings of a transformer whose primary winding 33 is mounted on the central core leg 35 between the magnetic shunt legs and H, and again the ballast is shown clamped together as a unit by the novel spring clamp H disclosed in the patent application, Serial No. 24,047, filed April 29, 1948, in the name of Gregory J. Ganley and assigned to the same assignee as the present invention.

Again as shown in Fig. 6, in order to prevent the leakage flux paths from having too low a reluctance, air gaps are formed by the notches 44 and 45 in the sides of the winding leg 35. In order to achieve a bridged gap in the portion of the core which is traversed by the leading secondary flux, the winding leg 35 is formed with an integral projecting tip 46 which cooperates with the outer yoke legs 42, as shown in Fig. 7, in minimizing the movement of the winding leg 35 with respect to the yoke legs 43 as well as establishing the air gaps 41 at the end of the central core leg 35.

The semi-forced core construction illustrated in Fig. 6 and shown in greater detail in Fig. 7 accomplishes, in essence, the same results as the core construction illustrated in Fig. 1 but has the advantage of allowing for more economical manufacture. The air gap 41 is maintained or gaged by the mechanical interference between the yoke legs 42 and the winding leg 35 at the tip 46. The normal vibration of the winding leg 35, which is due to the action of flux traversing the air gap 41, is prevented by this same mechanical interference.

To further reduce the noise level of the fluorescent lamp ballast 34, the lead end air gap 41 may be filled with a cement in accordance with the method described in the application, Serial No. 68,557, filed concurrently herewith in the name of Charles P. Hayes and which is assigned to the same assignee as the present invention. The grooves 48 in the outer yoke legs 42 form a header which facilitates the application and distribution of the cement. Similarly grooves 49 may be punched as part of the indentations 44 and 45 in the winding leg 35 to facilitate the use of the cement or filler.

Referring again to Fig. 8, it can be seen that the primary coil [0 and both secondary coils 8 and 9 are conductively tied together since they are all connected to the supply line 50. The other supply line 5| is connected to the primary coil l0 and the fluorescent lamps l2 and I4. Lamp I2 is connected to lead secondary 9 through the capacitor l3, and the lamp I4 is connected directly to the lag secondary 8; thus the voltage across lamp I4 is the voltage across the primary 1 0 plus the voltage of the secondary 3. Similarly, the voltage across lamp I2 is the sum of the primary voltage and the voltage due to secondary 3. These lamp voltages are those present before conduction through the lamps l2 and I4 begins. After conduction has once started, the lamp voltage is changed by the regulation of the circuit.

While there have been shown and described particular embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention, and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A magnetic core comprising, in combination, a central winding leg member and a pair of multiegged yoke members disposed on opposite sides of said winding leg member, one and leg of each of said yoke members being partially grooved, one end of said central winding leg member being similarly grooved, a pin of magnetic material cooperating with said grooves and making contact with said central winding leg member and said and legs of said yoke members so as to form a bridged gap between said winding leg member and said yoke members.

2. A core for a high reactance transformer type two-lamp fluorescent ballast comprising, in combination, a pair of four-legged yoke members, the outer legs of said yoke members being substantially longer than the inner legs of said yoke members, each of said inner legs being of substantially different widths, said yoke members being disposed with their legs extending toward each other, a generally straight central winding leg member, said winding leg member being disposed in relation to said yoke members so as to lie between them, one end leg of each of said yoke members being partially grooved, one end of said central winding leg being similarly grooved, a pin cooperating with said grooves and making contact with said central winding leg member and said end legs of said yoke members so as to form. a bridged gap between said winding leg member and said yoke members, said yoke members substantially surrounding said central winding leg member.

3. A magnetic core comprising, in combination. a central winding leg member and a pair of multi-- legged yoke members disposed on opposite sides of said winding leg member, one end leg of each of said yoke members having an integral stud extending therefrom, said studs making contact with said winding leg member so as to form a bridged gap between said winding leg member and said yoke members.

4. A magnetic core comprising, in combination, a central winding leg member and a pair of multilegged yoke members disposed on opposite sides of said winding leg member, a plurality of integral studs extending from one end of said winding leg member, each of said studs being contiguous with one end leg of each of said yoke members and cooperating with said end leg so as to form a bridged gap between said winding leg member and said yoke members.

5. A magnetic core comprising, in combination, a central winding leg member and a pair of multilegged yoke members disposed on opposite sides of said winding leg member, an integral projection extending from one end of said winding leg member, one end leg of each of said yoke members making contact with said winding leg member at said projection only so as to form a bridged gap 7 between said winding leg member and said yoke members.

6. A magnetic core comprising, in combination, a central winding leg member and a pair of multilegged yoke members disposed on opposite sides or said winding leg member, an integral projection extending from one end of said winding leg member, one end leg of each 01 said yoke members making corner contact with said winding leg member at said projection only so as to form a bridged gap between said winding leg member and said yoke members, the other end of said winding leg member being in abutting contiguous contact with a side of the remaining end leg of each of said yoke members.

'7. A magnetic core comprising, in combination, a central winding leg member and a pair of multilegged yoke members disposed on opposite sides of said winding leg member, an integral projection extending from on end of said winding leg member, one end leg of each of said yoke members making contact with said winding leg member at said projection only so as to form a bridged gap between said winding leg member and said yoke members, said end leg of each of said yoke members being formed so that the mechanical interference between said projection and said end legs aids in orienting and maintainim the position of said winding leg member in relation to said yoke members.

8. A magnetic core comprising, in combination, a central winding leg member and a pairof multilegged yoke members disposed on opposite sides of said winding leg member, an integral projection extending from one end of said winding leg member, one end leg of each of said yoke members making contact with said winding leg member at said projection only so as to form a bridged gap between said winding leg member and said yoke members, said end leg of each or said yoke members being formed so that the mechanical interference between said projection and said end legs aids in orienting and maintaining the position of said winding leg member in relation to said yoke members the other end of said winding leg memher being in abutting contiguous contact with the remaining end leg of each of said yoke members.

9. A magnetic core comprising, in combination, a central winding leg member and a pair of four legged yoke members disposed on opposite sides of said winding leg member, the outer legs of each or said yoke members being substantially longer than the inner legs of said yoke members, an integral projection extending from one end of said winding leg member, one end leg of each of said yoke members making contact with said winding leg member at said projection only so as to form a bridged gap between said winding leg member and said yoke members.

10. A magnetic core comprising, in combination, a central winding leg member and a pair of four legged yoke members disposed on opposite sides of said winding leg, the outer legs of each of said yoke members being substantially longer than the inner legs of said yoke members, an integral projection extending from one end of said winding leg member, one end leg of each of said yoke members making contact with said winding leg member at said projection only so as to form a bridged gap between said winding leg member and said yoke members, the other end of said winding leg member being in abutting contiguous contact with the remaining end leg of each of said yoke members.

11. A core for a high reactance transiormer type two-lamp fluorescent ballast comprising, in

combination, a pair of four-legged yoke members, the outer legs of each or said yoke members be substantially longer than the inner legs of said yoke members, each of said inner legs being of substantially different width, said yoke members being disposed with their legs extending toward each other, a generally straight central winding leg member, said winding leg member being disposed in relation to said yoke members so as to lie between them, an integral projection extending from one end of said winding leg member, one end leg 0f each of said yoke members making contact with said winding leg member at said projection only so as to form a bridged gap between said winding leg member and said yoke members, the remaining end of said winding leg member being in abutting contiguous contact with the remaining end leg of each of said yoke members, said winding leg member having notches in the vicinity of the ends of said inner yoke legs.

12. A fluorescent lamp ballast comprising, in combination, a magnetic core having a pair of four-legged yoke members, the outer legs of each of said yoke members being substantially longer than the inner legs of said yoke members, each of said inner legs being of substantially different widths, said yoke members being disposed with their legs extending toward each other, a generally straight central winding leg member, said winding leg member being disposed in relation to said yoke members so as to lie be. een them, a plurality of coils being mounted on said central winding leg member, said coils substantially filling the spaces between the legs of said yoke members, an integral projection extending from one end of said winding leg member, one end leg of each of said yoke members making contact with said winding leg member at said projection only so as to form a bridged gap between said winding leg member and said yoke members, the remaining end of said winding leg member being contiguous with the remaining end leg of said yoke members, said winding leg member having notches in the vicinity of the ends of said inner yoke legs, said yoke members substantially surrounding said central winding leg member, and clamping means associated with said members of said ballast maintaining as a single unit said members of said core.

13. A magnetic core for a high power factor ballast for separate leading and lagging current discharge lamp circuits comprising, a plurality of fiat stac'ked lamination layers each including at least two punchings of magnetic sheet material, one of said punchings being a substantially straight winding leg punching and the other being a yoke punching for completing a magnetic circuit between the ends of the winding leg punching, the yoke punching comprising two end members and two intermediate magnetic shunt members all parallel with each other and all integrally connected perpendicularly to at least one substantially straight member which is parallel to said winding leg punching, one of said end members having a recessed portion which embraces in c ose contact one end of said winding leg punching, the other end of said winding leg punching having an integral reduced cross section axially extending tip, the other end member having a small recessed portion within a larger recessed portion, said small recessed portion embracing in close contact the end of said tip, said larger recessed portion embracing loosely the tipped end of said winding leg punching with a gap therebetween, whereby the length of said tip gages the length of said gap and said tip constitutes a magnetic bridge across said gap, the magnetic shunt member nearest the tipped end of said winding leg punching being substantially narrower than the other one, and gaps between the ends of said magnetic shunts and said winding leg member.

14. A magnetic core for stationary electric induction apparatus comprising, in combination, a plurality of similar lamination layers of at least two magnetic sheet punchings each, said punchings in adjacent'layers being congruently superposed, one punching being a substantially straight winding leg member, another punching being a yoke member which has aligned spacedapart recesses which embrace respectively the opposite ends as distinguished from the sides of said leg member, one of said recesses being double with a maller one inside a larger one, a protruding tip on one end of said leg member fitted into said smaller recess for separating said larger recess from said end of said leg member so as to constitute a bridged gap and also force the other end of said leg member into intimatecontact with the other recess, and spaced high reluctance magnetic shunts between said yoke member and at least one of said leg members, the shunt nearest the tipped end of said leg member having the smaller cross section.

15. A magnetic core for stationary electric induction apparatus comprising, in combination, a plurality of similar lamination layers of at least two magnetic sheet punchings each, said punchings in adjacent layers being congruently superposed, one punching being a substantially straight winding leg member, another punching bein a yoke member which has a recess which embraces one end as distinguished from the sides only of said leg member, and an integral protruding tip on one of said members for forming a. bridged gap between the other end of said leg member and said yoke member and for forcing the first mentioned end of said leg member into intimate contact with said recess, and spaced high reluctance magnetic shunts of unequal cross section between said yoke member and at least one side of said leg member, the shunt nearest said bridged gap having the smaller cross section.

16. A magnetic core comprising, in combination, an elongated generally straight winding leg member and a pair of similar four-legged yoke members disposed on opposite sides of said winding leg member, the four legs of each yoke member comprising two end legs and two intermediate leakage flux legs, the end legs being longer than the leakage flux legs, one leakage flux leg being substantially wider than the other one, one end leg of each yoke member and an op posite side of the same end of said winding leg member having interfitting and contacting surfaces each of which surfaces extends in at least two different directions, the other end legs of each yoke member and an opposite side of the other end of said winding leg member having spaced generally parallel surfaces forming nonmagnetic gaps therebetween, the narrower leakage fiux leg being closest to the gapped end of said winding leg and the wider leakage flux leg being closest to the other end of said winding leg, the ends of said leakage flux legs being spaced from the sides of said winding leg mem- 10 ber so as to fcrm nonmagnetic gaps therebetween.

1'7. A magnetic core comprising, in combination, an elongated generally straight winding leg member and a pair of similar four-legged yoke members disposed on opposite sides of said winding leg member, the four legs of each yoke member comprising two end legs and two intermediate leakage flux legs, the end legs being longer than the leakage flux legs, one leakage flux leg being substantially wider than the other one, one end leg of each yoke member and an opposite side of the same end of said winding leg member having interfitting and contacting surfaces each of which surfaces extends in at least two different directions, the other end leg of each yoke member and an opposite side of the other end of said winding leg member having spaced generally parallel surfaces forming nonmagnetic gaps therebet'-..een, and a body of magnetic material forming a bridge across each of said gaps which bridge is of small cross-section relative to the cross-section of said gaps, the narrower leakage flux leg being closest to the bridged gap end of said winding leg member and the wider leakage flux leg being closest to the other end of said winding leg, the ends of said leakage flux legs being spaced from the sides of said winding leg member so as to form nonmagnetic gaps therebetween.

18. A magnetic core comprising, in combination, an elongated generally straight winding leg member and a pair of similar four-legged yoke members disposed on opposite sides of said winding leg member, the four legs of each yoke member comprising two end legs and two intermediate leakage flux legs, the end legs being longer than the leakage flux legs, one leakage flux leg being substantially wider than the other one, one end leg of each yoke member and an opposite side of the same end of said winding leg member having interfitting and contacting surfaces each of which surfaces extends in at least two different directions, the other end leg of each yoke member and an opposite side of the other end of said winding member having spaced generally parallel surfaces forming nonmagnetic gaps therebetween, said other end legs of said yoke members and said other end of said winding leg member also having integral contacting portions of small area relative to the area of said gaps forming a magnetic bridge across said nonmagnetic gaps, the narrower leakage flux leg being closest to the bridged gap end of sad winding leg member and the wider leakage flux leg being closest to the other end of said winding leg member, the ends of said leakage flux legs being spaced from the sides of said winding leg member so as to form nonmagnetic gaps therebetween.

WESLEY W. BROOKS.

REFERENCES CITED The following references are of record in the file of this patent:

. UNITED STATES PATENTS Number Name Date 1,783,050 Merrill Nov. 25, 1930 2,137,433 Wirz Nov. 22, 1938 2,269,978 Kronmiller Jan. 13, 1942 2,317,844 Boucher Apr. 27, 1943 2,382,638 Keiser Aug. 14, 1945 

